In the field of examining the pattern of a mask used for a semiconductor integrated circuit such as a reticle or a photomask, higher accuracy is being demanded along with the necessity of miniaturization of the pattern in recent years.
In general, the pattern examination is carried out by forming an observation image which is obtained by irradiating a laser beam or a charged particle beam to a mask, and a reference image which is obtained by performing calculation from design data of corresponding parts, and then comparing thus formed observation image and reference image to find out mismatched parts.
In order to realize the defect examination of high accuracy, the pattern on an observation image is required to accord with the corresponding pattern on a reference image correctly at parts where there is no defect.
Actually, in obtaining an observation image, there is raised a quantity of strain due to the speed unevenness in shifting a stage or problems in the optical system.
Problems in the optical system are as follows. In an optical system that obtains an image by oscillating a beam using an acoustooptic device (AOD) etc. and measuring the beam amount of the transmitted beam, there is raised a strain due to the influence of the AOD, and the degree of strain changes due to the influence of the thermal storage with time. Furthermore, there may be raised a strain in an obtained image due to the strain of a lens which is raised in the peripheral part and central part thereof, and the degree of strain changes due to the influence of the thermal storage with time. Moreover, in case the automatic focus is utilized so as to counter a quantity of warpage raised in a reticle, the focal length is made to fluctuate, and the degree of strain changes according to the fluctuation.
Under the circumstances in which the miniaturization of the pattern is being advanced in recent years, above-described strain, which has not been a problem and not been taken into consideration, cannot be ignored, and the necessity of compensating a strain comes to be indispensable.
As a system to compensate a strain which is raised at the time of obtaining an observation image, in Patent Document 1 and Patent Document 2, there is disclosed a system that measures the shift amount of a stage using a sensor, and compensates a reference image by utilizing the value of the shift amount.    Patent Document 1: JP 2003-121984-A    Patent Document 2: JP 2003-090717-A    Patent Document 3: JP 10-325806-A    Patent Document 4: JP 11-211671-A    Patent Document 5: JP 2000-348177-A
Non-Patent Document 1: Pattern classification (second edition), on pages 111 to 113, written by Richard O. Duda and others, translated by Morio Onoe, published by New Technology communications in 2003
Non-Patent Document 2: Pattern classification (second edition), on pages 120 to 125, written by Richard O. Duda and others, translated by Morio Onoue, published by New Technology communications in 2003